Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy into light. Improvements in these devices have resulted in their use in lighting fixtures as replacements for conventional incandescent and fluorescent light sources. LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. In addition, the energy conversion efficiency of LEDs has now reached the same level as that obtained in fluorescent light fixtures and promises to exceed even these efficiencies.
A single LED produces too little light to be used as a replacement for a conventional lighting source in most applications. Hence, a replacement light source must utilize a large number of individual LEDs. The packaging costs and reliability problems that result from having to use large numbers of individual LEDs present challenges that must be overcome if LED-based light sources are to reach their full potential as replacements for conventional light sources.
FIG. 1 (prior art) is a cross-sectional view of a portion of a prior art, phosphor-converted, LED light source. Light source 20 includes a plurality of LEDs of which LED 21 is typical. The LEDs are mounted on a printed circuit board 32 that includes a heat-spreading layer 33, an insulating layer 34, and a conducting layer that is patterned to provide electrical conductors such as conductor 35. The LEDs are mounted in depressions having reflecting walls 36 that re-direct light leaving the side surfaces of the LEDs such that the light leaves the light source in the vertical direction as shown by the arrows. The surface of heat-spreading layer 33 is typically covered with a reflecting material that redirects any light that is emitted in a downward direction into the upward direction. The LEDs are covered by a layer of phosphor 37 that converts a portion of the blue light generated by the LEDs to light in the yellow region of the optical spectrum. The combination of the blue and yellow light is perceived as “white” light by a human observer.
The LEDs include a light-emitting structure 22 that is deposited on a sapphire substrate 23. The light-emitting structure can be viewed as an active layer 24 that is sandwiched between an n-type GaN layer 28 that is deposited on substrate 23 and a p-type GaN layer 25 that is deposited over the active layer. The device is powered from contacts 26 and 27. Since p-type GaN has a very high resistivity, a current spreading layer 29 is typically deposited on the surface of layer 25. In the arrangement shown in FIG. 1, light is extracted through the top surface of the LED, and hence, the current spreading layer must be transparent. Typically, indium tin oxide (ITO) is used for the current spreading layer.
The electrical connections to the LEDs are provided by wire bonds, such as bond 31, that connect the contacts on the LEDs to corresponding contacts on a printed circuit board. The wire bonds present problems in terms of fabrication cost and reliability, particularly when the light source includes a large number of individual dies. The wire bonds are subject to failure both at the time of initial implementation of the bonds and later due to stresses between the phosphor layer and the encapsulated wire bonds. In addition, the wire bonds block a significant fraction of the light leaving the LEDs, as both the bond pads and the gold wire absorb light.
The arrangement shown in FIG. 1 provides good light capture with respect to the light leaving the sides of the LEDs. However, this aspect requires a more complex mounting substrate having reflective cups. The cost of the substrate increases the cost of the light source.
The arrangement shown in FIG. 1 has the advantage of providing good heat conduction because the entire bottom surface of the LEDs is in contact with the heat-spreading layer 33 of the printed circuit board. Heat removal is an important aspect of high-powered LED light sources, as the efficiency of the LEDs decreases with temperature. In addition, mechanical problems that arise from differences in the thermal coefficient of expansion between phosphor layer 37 and printed circuit board 32 become worse as the operating temperature increases.
The problems associated with wire bonds can be reduced by utilizing a flip-chip mounting scheme. FIG. 2 (prior art) is a cross-sectional view of a portion of another prior art light source that utilizes a flip-chip mounting scheme. Light source 40 includes a plurality of surface mounted LEDs. For the purposes of this discussion, a surface mounted LED is defined to be an LED in which both the p-contact and the n-contact are on one side of the LED, light being emitted primarily through an opposing surface of the LED, although some of the light may be emitted through the side surfaces of the LED. In the case shown in FIG. 2, the sapphire substrate 41 faces upward and the LEDs are connected to the mounting substrate by the contacts that are used to power the LEDs. Light is emitted through the sapphire substrate. The p-contact includes a mirror 42 that re-directs light striking the contact such that the light exits through the sapphire substrate or side surfaces of the LED. The mirror can also act as the current spreading layer thereby reducing or eliminating the need to use an ITO layer. While the ITO layer is not needed for current spreading in this arrangement, the ITO layer can still provide an ohmic contact with the p-GaN layer, and hence, a thin ITO layer may be included in the p-contact. Since light does not exit through the p-GaN layer, the p-contact can extend substantially over the entire active layer, and hence the problems of providing current spreading over the highly resistive p-GaN layer are substantially reduced. For the purposes of this discussion, the p-contact will be defined to extend over substantially all of the active layer if the p-contact overlies at least 60 percent of the active layer.
The n-contact and p-contact are bonded to corresponding traces 43 and 44, respectively, on the mounting substrate. These traces are patterned on an insulating layer 45 that overlies the heat-dissipating core region 46 of the printed circuit board. Suitable bonding materials that utilize solder, thermal compression bonding, or asymmetric conducting adhesives are known to the art. Novel asymmetric adhesives will be discussed in more detail below.
While the arrangement shown in FIG. 2 reduces the problems associated with the wire bonds, heat dissipation and the loss of light that exits through the sidewalls of the LEDs remains problematic. If the LEDs are mounted in reflective cups as described above, the cost of the substrate becomes a problem. Furthermore, the bonding process requires that the LEDs be pressed against the printed circuit board during the bonding process, and hence providing a pressure mechanism that can operate on all of the LEDs in a light source at once is problematic if the LEDs are in reflective cups.
An LED packaging arrangement is sought that allows light leaving the sides of flip-chip mounted LEDs to be emitted upwards without using reflective cups.